The number of synchronous communication satellites in use is substantially increasing. Fewer communication channels are available for use with synchronous satellites because most channels are already allocated for present uses. This requires the implementation of higher frequency channels, typically the Ku band. But at these higher frequencies there are greater problems in tracking the satellites due to adverse atmospheric or noise conditions.
Synchronous communication satellites are not totally fixed in one position but instead drift about a central location point. Such satellites frequency use station keeping thrusters to direct the satellite back to its assigned position. However, there is a continuing need to track the satellites to maintain an optimum signal strength for the received signal as well as to determined the position of the satellite for sending any commands to operate the thrusters.
The high frequency communication channels, such as 8 GHz and higher, suffer from atmospheric attentuation, particularly rainfall, to a much greater extent than the lower frequency channels. There can also be other noise interference with the signal which can interfere with the tracking activity.
Conventional tracking techniques include monopulse and step track. Monopulse systems, while very effective, are too expensive for many applications. Step track systems utilize algorithms to determine satellite position by sampling signal strengths in a predetermined pattern and directing the antenna to the maximum signal strength position. In the step track system the signal amplitudes between step positions must be determined very accurately. At higher frequencies of 8 GHz and higher, the variations in the satellite signal due to noise and/or atmospheric conditions is greater than the differences in signal amplitude between the step positions. This can prevent the accurate determination of the maximum signal amplitude position.
In view of the increasing use of high frequency communication channels and the requirement to accurately track satellites using such channels, there exists a need for a method and apparatus to provide tracking control for satellite antennas which can successfully operate despite the presence of substantial random noise and signal variations on the tracking signal.